The Bossier banner. [volume] (Bellevue, Bossier Parish, La.) 1859-1952, 1887-06-16
IRON GENERALIZATION.
How 1t Produces Changes In tho F1befs OF
the Metsls and weakens it.

How 1t Produces Changes in the Fibers Of
the Metsls and weakens IL.
Since q passenger train on the North
British railway plunged from the great
bridge over the Firth of Tay into the
dark waters below, one stormy Deccm-
Der night seven years ago, carrying ev-
cry living soul upon it to certain death,
there has been no railroad bridge dis
aster that has excited so much public
attention as that which occurred on the
Boston & Providence railroad at the
Bussey bridge near Boston. In both the
cause appears to have been structural
defect. This may also be said of the
Ashtabula accident. whose horrors, how
ever, were increased by the fierce work
of flames and the inclemencies of bit
terly cold winter night
Bridge accidents are considered by
all railroad men as incomparably the
worst to which travel by rail is exposed,
and as among the most difficult to
guard against absolutely. One source
OF danger was shown when the peculiar
effects of a locomotives hammer blows
upon a track were explained The
theory was advanced that a girder OF
the bridge had broken beneath the en-
gine's drivers, whose pounding upon
the effectively constructed bridge was
thus primarily the cause of the acct
dent. This brings up phase of rail-
road and bridge accidents which is at
present receiving great deal of atten-
ton. Capitalization in the material OF
iron bridges is an element of danger
now so well recognized that the Penn
sylvania Railroad Company has adopted
the definite policy of substituting stone
for iron bridges, and the same plan is
being followed by other railroad COM
panies.
The lifetime of an iron bridge is from
twenty to twenty-five years, and con-
slant care is necessary to insure its
safety. The Ashtabula bridge was
thirteen years old, and at the time it
fell beneath the weight of two engines
and heavy express train the theory
was commonly advanced that crystal
ization had impaired its original
strength. The two great moving causes
OF Capitalization are vibration and the
hammer blows Of the locomotive. which
gradually, by repeated impact, turn the
fiber OF iron or steel into a mass of
crystals. needing only some unusual
fSr-ssc 70 Nr. III A.
break apart. Add to this the oscillation
caused by the passage Of trains at
high rate of speed, and it will be seen
that an iron bridge is exposed to pecu-
liar perils, which can only be guarded
against by rigid inspection and con-
slant renewals.
Capitalization was one of several
theories advanced by Park Benjamin, a
New York engineer, to account for the
Tay bridge accident. He said:
General indications 60 to show that
the cause must be sought in an abnormal
condition of the structure, orrather one
which did not enter into the calculations
of the builders. Such condition would
be the deterioration of the metal by its
change from a fibrous to crystalline
state under repeated vibration. That
this occurs in railway bridges has been
vigorously disputed by many we'll
known engineers. On the other hand,
many examples are quoted by different
authorities to support the theory. APP
propos to this particular accident,
distinguished French engineer and iron
founder now in this country informs us
that he has known bars of iron made by
himself from Scotch pig to change from
tough fibrous to brittle crystalline
structure in travelling by rail only from
the north of France to Paris. This is, of
course. an extreme instance.
again, recent research has DEM
onstrated that because structure
withstands large quiescent load that
fact is little proof of stability under re-
peated shocks and vibrations. Metals
are believed to have life A bar, for
example, may stand a million vibra-
tons and break down at the million
and first, and yet the last shock may be
lighter than preceding ones. Attempts,
however, to reduce this law to
practical application have elected
an abundance of conflicting evidence;
but, nevertheless. it is well settled that
in no department of mechanics is an
extended course of actual experiments
ing more urgently needed or of graver
public importance.
The strength Of the Tay bridge at the
time of its building was such that it
was pronounced perhaps the most
remarkable structure in the world.'
Engineering, British magazine, speak
ing of its test. when five engines,
weighing $50 tons, were placed on a
single span, said: sThe result is the
complete establishment of this fact ESQ
important to the public, that the
bridge is strong out of all proportion to
its possible necessities. The load which
the structure is calculated to carry is
six times greater than that to which it
was subieetad ,,